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Subject: Re: Effect of sampling frequency for FFT
Date: Fri, 30 Oct 2009 22:19:05 +0000 (UTC)
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"Matt " <xys@whatever.com> wrote in message <hcf0h0$kqk$1@fred.mathworks.com>...
> "juho salminen" <jssalmi3@cc.hut.fi> wrote in message <hcev3g$lej$1@fred.mathworks.com>...
> 
> > Highter sampling frequency then, of course, reduces the time window size. 
> 
> By time-frequency duality, windowing in the time domain is equivalent to low-pass filtering the frequency spectrum. By decreasing the window-size you are applying a low pass filter to your spectral data of lower and lower cut-offs and hence greater and greater smoothing.

Sorry for inconvenience, what I meaned was, that actual FFT window size is always the same, let's say 500 points. But, when I sample artificial signal with highter sampling frequency, Amplitude of sine wave in FFT is much more accurate than with lower sampling frequency.

For example, 1. I sample some artificial noisy sine wave with 50Hz, and I collect 5000samples, 2. I sample some artificial sine wave with 200Hz, and I collect 5000samples, the second case gives more accurate amplitude results in FFT